Method of Producing Carbon-Encapsulated Metal Nanoparticles

ABSTRACT

A method of producing carbon-encapsulated metal nanoparticles, for example carbon-encapsulated magnetic metal nanoparticles, including providing a carbon-containing metal salt or organometallic compound in a reactor, for example a vessel having a restricted opening and decomposing the carbon-containing metal salt or organometallic compound, for example by heating, whilst maintaining carbon within the reactor to form carbon-encapsulated metal nanoparticles.

The present invention relates to a method of producingcarbon-encapsulated metal nanoparticles.

Carbon-encapsulated magnetic nanoparticles are important newnanomaterials (Peter J. Harris, Chapter 5, “Carbon Nanotubes and RelatedStructures”, Cambridge University Press, 1999). The carbon-encapsulatedmagnetic nanoparticles consist of magnetic nanoparticles (for example ofiron, nickel or cobalt) encapsulated within carbon nanotubes orfullerene-like or polyhedral graphitic cages. In either case, acompletely sealed carbon structure is typically found. The typicalparticle size is 10 to 500 nm.

Applications of carbon-encapsulated magnetic nanoparticles includehigh-density magnetic data storage, magnetic toners for use inphotocopiers, magnetic inks and ferrofluids (S. Subramoney, Adv. Mater.10, 1 557, 1998). The carbon coatings mean that the magneticnanoparticles are biocompatible and are stable in many organic media.Thus, carbon-encapsulated magnetic nanoparticles are candidates forbioengineering applications, for example drug delivery, biosensors,magnetic hyperthermia and magnetic contrast agents for MagneticResonance Imaging (A. A. Bogdanov, C. Martin, R. Weissleder, T. J.Brady, Biochim. Biophys. Acta, 1193, 212, 1994).

Because of the protective graphitic sheets encapsulating the magneticnanoparticles, the magnetic nanoparticles are protected from theenvironment and from degradation. In addition, the graphitic sheetsisolate the magnetic nanoparticles magnetically from one another. Thismeans that problems caused by interaction between closely spacedmagnetic bits are avoided.

Carbon-encapsulated magnetic nanoparticles have been produced by arcevaporation in the Huffman-Kratschmer chamber (T. Hayashi, S. Hirono, M.Tomita, S. Umemura, Nature, 381, 772, 1996; J. Henry, J. Scott and S. A.Majetich, Phys. Rev. B, 52, No. 17, 12564, 1995). This techniqueinvolves evaporation of a mixture of metal catalyst and graphite byelectrical arc discharge (typically 100-200 A) at extremely hightemperatures (above 3000° C.).

Laser ablation has also been used. This method producescarbon-encapsulated magnetic nanoparticles in a much higher yield thanarc discharge. However, although both methods can produce good qualityproducts, they are unsuitable for use on a large scale because of poorand irreproducible yields and presence of many carbonaceous by-products.

Carbon-encapsulated magnetic nanoparticles have been produced byChemical Vapour Deposition (CVD) by passing a carbon source (typically ahydrocarbon) over a supported metal catalyst. This method is low-costand only requires simple apparatus, and can be well controlled. However,this method cannot be used to produce carbon-encapsulated magneticnanoparticles on a large scale because of poor yields and the difficultyof separating the carbon-encapsulated magnetic nanoparticles from thesupporting materials (Z. Y. Zhong, H. Y. Chen, S. B. Tang, J. Ding, J.Y. Lin, K. L. Tan, Chem. Phys. Lett., 330, 47, 2000).

Carbon-encapsulated magnetic nanoparticles have also been produced bypyrolysis of non-graphitising carbon (P. J. F. Harris and S. C. Tsang,Chem. Phys. Lett., 293, 53, 1998). The non-graphitising carbon is firstimpregnated with a salt of the metal to be encapsulated. The driedproduct is heated to temperatures of 1800 to 2500° C. The encapsulatedproducts are similar to those prepared by the arc discharge method.However, the yield of product is low.

The present inventors have invented a new method suitable for largescale synthesis of carbon-encapsulated magnetic nanoparticles.

Accordingly, in a first aspect, the present invention provides a methodof producing carbon-encapsulated metal nanoparticles, comprising thesteps of:

-   -   providing a carbon-containing metal salt or organometallic        compound in a reactor; and    -   decomposing the carbon-containing metal salt or organometallic        compound to form carbon-encapsulated metal nanoparticles.

Preferably, carbon is maintained within the reactor duringdecomposition. The carbon may be in elemental or molecular form. Carbonshould be maintained at a vapour pressure at the reaction site adequateto ensure that carbon-encapsulated metal nanoparticles are formed. It isnot necessary for all carbon contained in the metal salt ororganometallic compound to be maintained within the reactor.

Preferably, decomposition is carried out in a reactor having arestricted opening. The reactor thus substantially confines the carbonwithin the reactor where it reacts to form carbon-encapsulated metalnanoparticles. It is preferred to provide a restricted opening so thatby-products of decomposition can escape from the reactor.

Preferably, the reactor is a tube having one sealed end and one end witha restricted opening. The tube should have a large length to diameterratio, for example a ratio of 30 or higher. This also assists inconfining carbon within the reactor. Alternatively, the reactor may be aflask having a restricted opening.

Preferably, a means of escape for elements other than carbon and metalreleased during decomposition (e.g. oxygen and nitrogen) is provided.Suitably, a gas flow to carry away such elements is provided.

However, a unidirectional gas flow across the reaction site ispreferably prevented. This assists in preventing carbon from beingcarried away from the reaction site. Where a flow of gas is providedduring reaction, the flow should be directed towards the restrictedopening of the reactor.

Preferably, the carbon-containing metal salt is decomposed under aninert gas atmosphere. This prevents formation of metal oxides and carbondioxide. Preferably, a flow of inert gas is provided.

Suitably, the inert gas is argon. If nitrogen is used as an inert gas,some incorporation of nitrogen into the product may be found.

Preferably, the carbon-containing metal salt is decomposed by heating.Alternatively, the salt may be decomposed by irradiation.

Preferably, heating is carried out at a temperature of 700 to 1500° C.More preferably, heating is carried out at a temperature of 700 to 1200°C. A temperature below 700° C. favours the formation of amorphouscarbon. A temperature above 1500° C. leads to rapid decomposition whichmay result in agglomeration of the metal to form large chunks. Suchchunks are not normally catalytically active.

Preferably, the metal is iron, nickel, cobalt, ruthenium, osmium,rhodium, iridium, palladium, platinum, a lanthanide or uranium. Morepreferably, the metal is a magnetic metal.

Preferably, the carbon-containing metal salt or organometallic compoundcontains at least 5 carbon atoms per metal atom.

Preferably, the carbon-containing metal salt is a carboxylic acid metalsalt. Suitably, the carbon-containing metal salt is a stearate.

In a second aspect, the present invention relates to carbon-encapsulatedmetal nanoparticles produced by a method as described above.

The invention will be further described with reference to the Examplesand as illustrated in the Figures, in which:

FIG. 1 shows a TEM image of an encapsulated nanoparticle produced inExample 1.

FIG. 2 shows an SEM image of the carbon nanotubes produced in Example 1.

FIG. 3 shows a TEM image of two carbon nanotubes produced in Example 1.

FIG. 4 shows an X-ray diffraction profile of the product of Example 1.

FIG. 5 shows an SEM image of encapsulated iron nanoparticles produced inExample 4.

FIG. 6 shows a TEM image of an encapsulated Fe₃C nanoparticle producedin Example 4.

FIG. 7 shows X-ray diffraction and Raman spectra of the product ofExample 4.

EXAMPLE 1

Nickel stearate (about 1.5 g, fine green powder) was evenly distributedin a quartz reactor having the shape of a tube of length 500 mm anddiameter 11 mm with a first sealed end and a second end with a smallopening of diameter 4 mm. The reactor was pumped to vacuum and filledwith argon. The reactor was slowly introduced into a tubular furnace oflength 800 mm and diameter 40 mm. The furnace was preheated to 800 ° C.and a flow of argon (1 to 2 l/min) was passed through the furnace. Thesmall opening of the reactor was directed towards the flow of argon. Thereactor was heated in the furnace for 10 mins and the furnace was thencooled to room temperature. The produced was collected at roomtemperature.

During loading of the reactor into the furnace the nickel stearatechanged from green to black in colour. Some gas passed out of thereactor during heating. The product (a fine black powder) was examinedby SEM and TEM and was found to consist of carbon nanotubes andcarbon-encapsulated nickel nanoparticles in fullerene-like or polyhedralgraphite cages.

The carbon encapsulated nickel nanoparticles (FIG. 1) were found to havea typical diameter of 30 to 150 nm. The graphite layers were wellcrystallised with 10 to 50 layers.

The carbon nanotubes (FIG. 2) were found to have a typical internaldiameter of 10 to 30 nm, although some much finer tubes of diameter lessthan 5 nm and larger tubes of diameter larger than 50 nm were alsoobserved. TEM observations (FIG. 3) indicated that most nanotubes weremulti-walled with 10 or more graphene layers. Many nanotubes also hadnickel nanoparticles encapsulated inside.

No naked nickel nanoparticles were observed under intensive and repeatedmicroscope observations, indicating that substantially all the nickelnanoparticles produced were encapsulated either in the fullerene-like orpolyhedral graphitic cages or in carbon nanotubes. The yield ofencapsulated nickel product was thus approximately 100%.

Electron diffraction and X-ray tests (FIG. 4) confirmed that theencapsulated nickel nanoparticles were pure nickel nanocrystals and thatthe surrounding carbon was well crystallined graphite.

Elemental analysis indicated C: 74% (corresponding to Ni: 26%).

EXAMPLE 2

The method of Example 1 was repeated at 1000° C. Examination of theproducts by electron microscopy indicated that they were similar instructure to the products of Example 1, but that the graphite layerswere even better crystallined. This indicates further that a highertemperature increases the degree of graphitisation.

EXAMPLE 3

The stability of the products of Example 1 was tested by heating asample in air to 400° C. in a quartz vial and cooling it to roomtemperature over 12 hours. No weight loss or colour change was observed.This indicates that the products had good thermal stability and thatthere was no amorphous carbon in the products, since this would haveburnt at a temperature of 355 to 400° C. to give a weight loss of thesample. It also indicates that there were no naked nickel particles inthe sample since these would have been oxidised. The good stability isalso confirmed that no degradation has so far been observed for productexposed to air for over 10 months.

EXAMPLE 4

The method of Example 1 was repeated using iron stearate rather thannickel stearate. The iron stearate was ground using a pestle and mortar.It was found that only carbon-encapsulated iron nanoparticles wereproduced (FIGS. 5 and 6). No naked iron nanoparticles were produced. Theproduct contained a small proportion of carbon nanotubes (estimated bySEM as less than 1%).

X-ray examination (FIG. 7) indicated that the encapsulated species werepure iron nanocrystals and iron carbide (Fe₃C) nanocrystals. The yieldof encapsulated product was thus approximately 100%.

EXAMPLE 5

The method of Example 1 was repeated using cobalt citrate rather thannickel stearate. The product was a mixture of carbon-encapsulated cobaltnanoparticles and naked cobalt particles.

COMPARATIVE EXAMPLE 1

Nickel stearate was heated to 800° C. in a boat under argon atmosphere.Naked nickel particles were produced.

COMPARATIVE EXAMPLE 2

Example 1 was repeated using a reactor having the shape of a tube oflength 500 mm and diameter 11 mm with both ends open. Naked nickelparticles were produced.

The high yields of encapsulated product achieved in the Examples meanthat these methods are suitable for bulk synthesis ofcarbon-encapsulated magnetic nanoparticles and carbon nanotubes.

The method of the Examples does not produce naked metal nanoparticleswhich are typical by-products in arc discharge. Further, thetemperatures used in the Examples are much lower than those required forarc discharge. The apparatus used in the Examples is also much simplerthan the arc discharge chamber.

The method of the Examples gives a much higher yield than thenon-graphitising carbon method. Again, the temperatures used in theExamples are much lower than those required for the non-graphitisingcarbon method.

The method of the Examples does not require the separate carbon source,catalyst and supporting materials of the CVD method. Instead, thestearate salt provides these three functions.

Comparison of the Examples and Comparative Examples show that to achievegood results argon must not be allowed to flow freely over the metalsalt. It is believed that confinement of atomic carbon near the reactionzone is necessary for formation of the desired product. Use of a longtubular reactor also assists in confining carbon. The reactor used inthe Examples has a length to diameter ratio of about 46.

Whilst the applicants do not wish to be bound by this theory, theybelieve that the large carbon to metal ratio of nickel stearatecontributes to the good results achieved in the Examples.

In nickel stearate, the metal content is 9.4 wt % and the carbon contentis 66 wt %. The metal content falls within the typical metal loadingrange for heterogeneous catalysts (5 to 10%). In contrast to aheterogeneous catalyst, nickel stearate provides a medium in which thedispersion of nickel atoms is uniform at a molecular level.

Although the apparent ratio of carbon to nickel is 36 to 1, theinventors believe that the effective ratio is much higher. This isbecause as nickel atoms are formed by thermal decomposition of the saltthey readily agglomerate to give particles, and only the nickel atoms onthe particle surface are catalytically active. For example, a sphericalnickel particle with a diameter of 20 nm contains about 6.4×10⁵ atoms ofwhich only about 3.7% are on the surface. This increases the effectivecarbon to nickel ratio to about 1000 to 1. The ratio increases as theparticle size increases.

The inventors believe that the high ratio of carbon to nickel withinnickel stearate means that under appropriate conditions thermaldecomposition of the molecule will produce nickel nanoparticles ascatalytic seed and sufficient carbon atoms to provide feedstock for thegrowth of carbon nanostructures.

1. A method of producing carbon-encapsulated metal nanoparticles,comprising: providing a carbon-containing metal salt or organometalliccompound in a reactor; and decomposing the carbon-containing metal saltor organometallic compound whilst maintaining carbon within the reactorto form carbon-encapsulated metal nanoparticles.
 2. A method as claimedin claim 1, wherein the reactor is a vessel having a restricted opening.3. A method as claimed in claim 2, wherein the vessel is a tube havingone sealed end and one end with a restricted opening.
 4. A method asclaimed in claim 1, wherein a unidirectional gas flow across thereaction site is prevented.
 5. A method as claimed in claim 1, whereinthe carbon-containing metal salt or organometallic compound isdecomposed under an inert gas atmosphere.
 6. A method as claimed inclaim 5, wherein the inert gas is argon.
 7. A method as claimed claim 1,wherein the carbon-containing metal salt is decomposed by heating.
 8. Amethod as claimed in claim 7, wherein heating is carried out at atemperature of 700 to 1500° C.
 9. A method as claimed in claim 1,wherein the metal is iron, nickel, cobalt, ruthenium, osmium, rhodium,iridium, palladium, platinum, a lanthanide or uranium.
 10. A method asclaimed in claim 9, wherein the metal is a magnetic metal.
 11. A methodas claimed in claim 1, wherein the carbon-containing metal salt ororganometallic compound contains at least 5 carbon atoms per metal atom.12. A method as claimed in claim 1, wherein the carbon-containing metalsalt is a carboxylic acid metal salt.
 13. A method as claimed in claim12, wherein the carbon-containing metal salt is a stearate or a citrate.14. Carbon-encapsulated metal nanoparticles produced by a method asclaimed in claim 1.